This invention relates generally to magnetic resonance imaging (MRI). MRI is a non-destructive method for the analysis of materials and is an approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
MRI is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During an MRI experiment, each nuclear spin responds to four different effects: precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MRI experiments, a combination of these effects occurs periodically.
Patients with many types of common implants, including pacemakers, defibrillators, guide wires, ablation catheters, and neurostimulators, are not eligible for MRI because of the risk of tissue heating due to coupling between the RF excitation and their device. Sometimes patients with dangerous implants are subjected to MRI scans, because the danger is not recognized for various reasons. Such scans may cause injury or death.